Vehicles, including wheeled vehicles, are typically suspended to absorb shock encountered while traversing uneven terrain. Wheeled vehicles usually include one suspension assembly per wheel so that each wheel may absorb shock independently. In many cases each such suspension assembly comprises both a spring portion and a damping portion. The spring portion may consist of a mechanical spring, such as a wound helical spring, or it may comprise a pressurized volume of gas. Gas is often used because it is light weight. Unlike typical simple mechanical springs, gas springs have non-linear spring rates. Compound mechanical springs may also have non-linear rates. A single gas spring has a spring rate that becomes highly exponential at compression ratios greater than about sixty percent. As a practical matter that can mean that a shock absorber including a gas spring can becomes very stiff just past the middle of its compressive stroke. Such excess stiffness over an extended length of the stroke is often undesirable (e.g. harsh riding vehicle).
In performing the dampening function, the damping mechanism of a shock absorber also creates resistance of the shock absorber to movement (e.g. compression and/or rebound). Unlike the spring which resists based on compressive displacement, fluid dampers usually have resistance to movement that varies with displacement rate (i.e. velocity). That may be disadvantageous because low velocity (i.e. low frequency) high amplitude shocks may compress the spring while the damper offers little resistance. In such cases the shock absorber may compress beyond a desired point because the damper did not contribute to shock compression resistance.
What is needed is a shock absorber dampener that offers resistance to movement as a function of axial displacement. What is needed is a suspension dampener that is relatively compliant at low axial displacement and progressively more resistant to movement at higher displacements. What is needed is a suspension (e.g. shock absorber, fork) having a gas spring with good low displacement resistance and more compliance at greater compression ratios. What is needed is a shock absorber having a gas spring and a dampener that can be tuned together to yield optimized shock absorber force travel velocity characteristics.